1. Field of the Invention
The present invention relates to a light source that emits light whose spectral characteristics are different by switching between an external electrode lighting mode and an internal electrode lighting mode.
2. Description of the Related Art
Conventionally, technology has been proposed where invisible information is obtainable as an image signal by printing invisible information on a specific image—e.g., with a material that transmits visible light and absorbs infrared light—and reading, with an image sensor sensitive to infrared light, light reflected when infrared light is irradiated onto the image.
In applying the aforementioned technology to an image reading device, how to add, as a simple configuration, a configuration for invisible information reading to a configuration for ordinary image information reading has become a problem.
Until now, in order to read both visible information and invisible information, reading has been conducted by switching between a visible light reading mode and an infrared light reading mode by using a halogen lamp as the illumination light source, using the infrared light components that a halogen lamp inherently has and switching optical filters inserted on a midway optical path, as disclosed in JP-A-6-141145.
Incidentally, in recent years, it is becoming more and more common to use a noble gas fluorescent lamp instead of a halogen lamp as the light source for ordinary image information reading, with the purpose of reducing power consumption and improving reliability.
However, because noble gas fluorescent lamps include practically no infrared light in the components of the irradiation light thereof in ordinary lighting conditions, they cannot be used as they are for invisible information reading, it is necessary to add a separate light source such as an infrared LED, and a problem arises in terms of cost and disposed space.
With respect to this problem, the inventors of the present application have proposed intensifying the infrared light component included in spectral characteristics of the illumination light by switching the lighting modes of a noble gas fluorescent lamp, as described in JP-A-2000-174984.
As one example thereof, the inventors have proposed an image reading device that irradiates light onto a target and reads the light reflected therefrom, the device including: an airtight container having disposed therein phosphor materials that emit light by ultraviolet rays which are radiated due to discharge; a pair of internal electrodes disposed inside the airtight container; and a pair of external electrodes disposed outside the airtight container, wherein the amount of the infrared component is switched by switching between a mode that causes a discharge between the internal electrodes and a mode that causes a discharge between the external electrodes.
In the mode that causes a discharge between the external electrodes, the discharge is not concentrated at a specific place because the discharge path is formed from a dielectric material such as glass. Thus, an impulse discharge of an extremely short amount of time is ubiquitously generated. As a result, ultraviolet light, which has a high energy, becomes the main component of the components of light emitted from xenon atoms of gas, and it becomes easy to excite the phosphors to emit visible components.
With respect thereto, in the mode where a discharge is caused between the internal electrodes, a dielectric material is not intervened on the discharge path, but a positive column is continuously joined between both electrodes. As a result, among the components of light emitted from the xenon atoms in the gas, the ratio of infrared light, which has a low energy, rises and the infrared component is directly emitted to the outside without exciting the phosphors. The present inventors actually made prototypes of lamps having these two electrodes and confirmed that the emitted light components are switched.
However, in the mode where the lamp is lighted by the external electrodes, the new problem arises that the internal electrodes sustain damage due to the discharge from the external electrodes.
As a countermeasure for a blackening phenomenon including this damage, in JP-A-5-144412, the blackening phenomenon is reduced by incorporating mercury in an internally sealed gas with respect to an internal electrode type. It has also been proposed to fill deuterium gas in a gas discharge display panel that has a structure similar to that of a noble gas fluorescent lamp.
However, when switching between the internal electrodes and the outer electrodes, the blackening phenomenon becomes worse than in the case of standard internal electrodes because the drive electric potential of the external electrode lighting mode that discharges through a dielectric material such as glass directly acts on the internal electrodes.
Also, as a light source having internal electrodes and external electrodes, there is a proposal for a structure in JP-A-2000-106146, but this is not a light source that switches between and lights two electrodes.